Differential motion conveyors are used to convey many types of products, for example, snack foods or the like. Snack food manufacturers may utilize differential motion conveyors to convey product from cooking process equipment to packaging equipment. Differential motion conveyors employ conveying forces having substantially only horizontal components unlike vibratory conveyors in which the conveying forces have both horizontal and vertical components. Since little or no vertical force components exist, it is generally perceived that differential motion conveyors handle fragile material such as snack foods in a gentler manner and therefore have less product breakage and build-up of food particles or flavoring material on the conveying surface than do vibratory conveyors.
In a differential motion conveyor, the conveying surface is displaced from a point of origin to a point of maximum deflection at one speed and returned to the point of origin at a faster speed. Particles on the conveying surface are subject only to the normal force due to gravity and to the coefficient of friction between the particle and the conveying surface. When the acceleration of the conveying surface, reacting with the mass of the particle, is greater than the normal force (Fn) multiplied by the static coefficient of friction (μs) of the particle, the particle slips relative to the conveying surface. When the acceleration of the conveying surface reacting with the mass of the particle is less than the normal force (Fn) multiplied by the kinetic coefficient of friction (μk) of the particle, the particle stops slipping and moves with the conveying surface.
During the slower forward speed portion of the conveying cycle, the acceleration of the conveying surface, reacting with the mass of the particle, is less than Fn×μk, so the particles move with the conveying surface. During the faster return speed portion of the conveying cycle, the acceleration of the conveying surface, reacting with the mass of the particle, exceeds Fn×μs, so the particles slide on the conveying surface, leaving them in an advanced position relative to their starting position on the conveying surface. Therefore, there is a net movement of the particles on the conveyor surface in the direction corresponding to the slower portion of the conveying cycle. Shifting the point of the velocity profile pattern of the conveying surface 180° with respect to the position of the drive arm (i.e., a faster forward speed portion of the cycle followed by a slower return speed) reverses the direction in which the particles are being conveyed.
The average operating speed of the conveyor is influenced by how well the conveying surface velocity is controlled, particularly during the slow portion of the conveying cycle. Intuitively, higher cycle speeds should yield higher feed rates, but then the conveyor becomes more sensitive to changing velocity patterns, resulting in greater accelerations than may be desired during some of the conveying portion of the cycle, resulting in feed rate inefficiencies. Slippage between the particles being fed and the conveying surface is also impacted by the coefficients of friction as previously noted. As a result, some materials feed better than others, but the coefficients of friction typically are not a major influence on the potential feed rate capabilities of the conveyor. Also, it is noted that for any given operating speed, the ratio between the slower portion of the cycle and the faster portion is important. Generally, the time difference between the two portions of the cycle can be optimized for the most efficient feed rate.
The remaining factor influencing feed rate at any given operating speed is the conveyor stroke (displacement). At any operating speed, feed rate is proportional to the conveyor stroke, the longer the stroke the greater the feed rate. The limiting factors on stroke include mechanical and dynamic design considerations, power usage requirements, isolation efficiency, and overall cycle speed control capability.
Prior art differential motion conveyors include machines that utilize complex mechanical drive systems to generate the required differential motion patterns of the conveying trough member. One such mechanical drive falls within a class type known as “four shaft” differential motion conveyor drives. These drives employ first and second pairs of counter-rotating drive shafts, with one pair operating at twice the speed of the first pair. Eccentric weights are mounted on each pair of drive shafts such that out-of-balance forces are generated as the weights rotate. The weights on the higher speed shaft are about one-third of the weight of the slow speed shaft to produce the desired differential motion. The counter-rotating shafts are synchronized in an attempt to cancel lateral force components generated, while the axial force components cyclically subtract from one another in one half cycle of rotation of the slow speed shaft pair, and cyclically add together during the remaining half cycle of rotation. During the half cycle wherein the axial force components subtract from one another, the conveyor trough is displaced in one direction at a certain velocity and is returned during the half cycle wherein the axial force components add together, resulting in a greater velocity. Particles placed on the conveying member will therefore be fed in the direction corresponding to the slower displacement portion of the cycle as previously explained. Characteristic of such prior art conveyors is that their eccentric weight systems are synchronized such that the “shaft pairs” are in phase with each other at some point of rotation within the cycle.
One drawback to “four shaft” conveyor drives is their high cost relative to the modest throughput capabilities obtained. Ongoing preventative maintenance is a must since, like any other machine having belts, gears, bearings and other wear parts, they might fail at an inopportune time. The present invention seeks to incorporate and improve upon the smooth harmonic motion velocity patterns of such conveyors, while substantially reducing manufacturing cost and reducing other “after installation” costs.
U.S. Pat. No. 5,938,001 to Turcheck et al., discloses a “four shaft” conveyor drive, whereby the eccentric weights are synchronized such that the “shaft pairs” are out of phase with each other during the rotation cycle. Such phase shift tends to make the conveying member's velocity pattern more linear, reducing slippage of the particles during the slower portion of the conveying cycle. The resultant conveying member velocity pattern is said to provide up to 50% greater throughput compared to previous “four shaft” conveyor drive designs. This is said to be accomplished without any departure from the size, number of weights, and operating speeds typically found in such drive designs.
Other prior art differential motion conveyors include various mechanical and electromagnetic motor drives to produce the required differential velocity patterns for conveying. U.S. Pat. No. 5,351,807 to Svejkovsky utilizes a universal joint driven off-axis to generate differential motion. However, this construction may be limited in size due to scale up constraints, and it requires the use of a hydraulic brake damper to eliminate inertial load backlash at the speed transition points. The potential problem of a hydraulic oil leak in a food-handling environment from a sanitary perspective is undesirable. Also, the design requires that the base members be solidly connected to the floor, precluding the possibility of suspension mounting the conveyor, which is preferred by many food plant operators.
U.S. Pat. No. 3,253,700 to Allen utilizes elliptical gears to generate differential motion. This configuration also requires some form of mechanical damping to eliminate inertial load backlash and to reduce mechanical noise and resultant gear tooth damage. Hydraulic fluids and gear oil required for this drive also raises sanitary installation issues in food handling environments. This type of conveyor also requires that the base members be solidly connected to the floor, making suspension mounting difficult or ruling it out as an installation option.
U.S. Pat. No. 5,409,101 to Ahmed et al. covers a method, utilizing an electric or electromagnetic motor, to generate differential motion velocity patterns for a conveyor by supplying cyclic non-uniform power to the motor. The linear motor disclosed in this patent is relatively complex, is relatively expensive to make, and requires relatively high input power to operate, at an ongoing higher operating expense.
Other motor driven differential motion conveyor designs include U.S. Pat. No. 5,794,757 to Svejkovsky that discloses a motor and crank arrangement for reciprocating a conveyor tray. In one embodiment of this patent, the motor controller operates the motor shaft first at one speed, corresponding to the slower forward half cycle of the conveying member, and then at a faster speed as the crank arm reverses the direction of the conveying member during the second half cycle of operation. A rather sophisticated and complex counterbalance system is used in an attempt to neutralize the effects of inertial load backlash at the motion reversal transition points as the motor speeds up and slows down.
Studies by the present inventors have shown that it would not be possible to optimize the differential motion velocity patterns required for efficient operation and maximum throughput by simply operating the motor first at one speed during one half cycle and then at a second higher speed during the remaining half cycle. Also, the cranks, crank assemblies, linkages and multiple counterweights compensating for unwanted vibration and noise are costly to manufacture and costly to maintain. Further, it would not be practical to suspend the apparatus, as it must be connected to a firm foundation to operate correctly. It may also be difficult to reverse the direction of feed of the conveyor by simply inverting the motion pattern due to the complex nature of the counterbalance system.
U.S. Pat. No. 6,415,911 to Svejkovsky is said to eliminate the need for the crank assemblies and complex counterweight systems and associated linkages disclosed in the U.S. Pat. No. 5,794,757 patent, by reversing the direction of rotation of the drive motor during one-half cycle of conveyor operation. While this improvement may address the issues of design and maintenance costs, it has reduced electrical power operating efficiencies because the motor must stop and reverse itself in such a way as to not disturb the smooth differential motion velocity patterns required for the conveyor to operate in an efficient manner. This design does not address the issues of product feed rate optimization, suspension mounting of the conveyor, or simple reversal of feed direction of the conveyed product.
U.S. Pat. No. 5,850,906 to Dean discloses a motor-driven differential motion conveyor wherein the motor is said to rotate at a constant speed to displace the conveying member during the slower speed portion of the conveying cycle. An electro-magnetically controlled variable viscosity clutch/brake assembly, in conjunction with a programmable controller and a position sensor, releases the conveying assembly's drive shaft from the motor drive shaft, allowing the conveying assembly to reverse direction. The restoring force from the conveyor's spring system then drives the conveying assembly in the reverse direction at a higher speed than the motor drove it, to produce the differential motion velocity pattern. The speed during the return portion of the half cycle is determined by the natural frequency of the conveyor's mass/spring system. At the end of the fast portion of the cycle, the clutch/brake re-engages, and the conveying member is again driven in the opposite direction at the slower speed, as the cycle repeats.
Also disclosed is a method to set the relationship between the timing of the first and second half cycles of operation as was determined through computer simulation. The direction of feed is switched by noting the position of the conveying member and engaging the clutch/brake 180 degrees out of phase with where it was in the original feed direction. Although the transitions between clutch/brake engagement and disengagement may be made smoother by allowing some slippage as the electromagnetic field is turned on and off to the clutch/brake, the machine essentially operates at two speeds and may not be able to generate the most efficient differential motion velocity patterns for optimum feed rate. Also, the conveyor would be difficult to suspend, since a solid mounting foundation is required. In addition, the required spring system might make the design more costly to manufacture and sell.